1. Field of the Invention
This invention pertains generally to photosensor devices, and more particularly to a vacuum sealed photosensor device adapted for scalable production.
2. Description of Related Art
Existing technologies for fabricating large-area photosensors, which have not been significantly improved since the 1960s, are outdated, expensive, low-quality, and labor intensive. FIG. 1 depicts a typical large-area photomultiplier tube (LAPMT) based on vacuum tube technology and dynode electron multipliers that are essentially hand-made, expensive and very problematic to produce in sufficiently large quantities. The structures of these existing devices suffer from numerous drawbacks, many of which arise from their “ship-in-a-bottle” type of design, in which all the elements of the device are retained, interconnected, and supported within a single enveloping glass-tube housing.
The LAPMT shown in FIG. 1 has a configuration whose manufacture is intrinsically labor-intensive, with the glass bulb and dynode column each accounting for about one half of the manufactured cost of an LAPMT. The complexity and cost of the bulb portion is significantly influenced by the necessity of having a long dynode column leading into the spherical bulb portion. The configuration of existing devices requires that the bulb and the dynode column are both created in a substantially handmade manufacturing process.
In addition, it should be noted that an LAPMT has a closed topology, and the photocathode formation process actually takes place within an assembled LAPMT. In this way every LAPMT can be considered in some manner to be its own “factory” whose “tools” for creating the photocathode remain in the LAPMT forever.
One class of LAPMT devices called Hybrid Photon Diodes replaces the dynode chain in a PMT with a semiconductor electron detector, typically with an ordinary Avalanche Photo Diode (APD) placed within the vacuum enclosure. HPDs are very expensive to produce for a number of reasons. First, there is the high cost of the Avalanche Photo Diodes themselves, which cannot merely comprise a multi-cell Geiger-mode APD, because of their large dead area (approximately 50%) for direct photoelectron detection. For application in HPDs, ordinary APDs which have no dead area, however, require that the passivation surface layer be removed. Second, there is a need for high voltage supplies to an ordinary APD. Third, there is a need to avoid contamination of ordinary APDs with alkali metals used for photocathode fabrication. The direct use of ordinary APDs is thus fraught with numerous drawbacks, while they are also very sensitive to even slight amounts of light overexposure, and in other ways are too fragile for a number of applications. Still further, an APD provides only a very low gain (thousands), with an output signal that requires careful shielding and significant amplification using expensive preamplifiers.
Furthermore, existing LAPMT device solutions are subject to a number of serious performance problems, including but not limited to the following: (1) very low photoelectron collection efficiency, such as only approximately 70%, which is often unlisted on manufacturer data sheets; (2) modest quantum efficiency; (3) non-uniform quantum efficiency; (4) high sensitivity to geomagnetic fields; (5) complicated and expensive mounting options; (6) highly fragile packaging which was dramatically demonstrated in the Super Kamiokande disaster; and (7) lack of single-photon resolution.
Major manufacturers of PMTs have either discontinued their production, or appear to be considering doing so. As a result, the fields of astroparticle and particle physics, as well as many other applications could become “stranded” without availability of this core detector component. The available devices, such as silicon detectors, for instance like the avalanche photo diodes (APDs) utilized for the readout of crystals in the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) at CERN in Geneva, are considered too small and costly for use in experiments requiring a very large sensitive area.
Accordingly, a need exists for a photosensitive device which approaches that of an ideal detector without the attendant implementation complexities and costs. The present invention overcomes these issues and provides an inexpensive and scalable photosensor element which can be readily mass-produced.